The composition of the cloud also prevents the hyperdrive emitter on the Prometheus from transferring power at an even rate, which makes a hyperspace window much too unstable to enter. It also prevents Prometheus' sublight engines from engaging.
In 2004, Major Samantha Carter was trapped in the cloud, as well as the Prometheus and a mysterious alien spaceship. She discovered that the only way out was to dial down the power flow to the hyperdrive emitter to cause a partial shift into hyperspace, essentially taking Prometheus out of the cloud's space-time. It was just enough to eliminate the cloud's effects on the sublight engines, enabling the Prometheus to exit the cloud. (SG1: "Grace")
In 2009, after encountering the aliens again, Carter realizes that the entire encounter was a test. She asks the alien if the alien ship getting stuck was part of the test and he admits that it actually wasn't: the aliens were simply trying to close the distance so they could beam away the crew. The aliens didn't expect Prometheus to launch escape pods and they nearly didn't take the risk of entering the cloud after them. (SGA: "Pleasure Cruise")